Lead removal in the electrowinning of chromium



1959 M. c. CAROSELLA ETAL 2,872,395

LEAD REMOVAL IN THE ELECTROWINNING OF CHROMIUM Filed Jan. 27,1958

Hake-up NH 50 Crystal:

Iron-Chromium Alloy Reduced Anolyto Chrome Alum 1 Preclpnfafe Minor portion 15 Llquor 3 mm Residue Ammonia bearing W z a Chrome Solufion Filfrgte m Conmngng Mflallu; 16 Impurmes Basic Chrome Sulfate Precipirafe Cflholyfe Overflow H20 W Ba Salf BaSaH's EM Ba+Pbpp 4 Chromium Mefal NH bearing U Solution Cr sfallizer Crude Ba Self x Ba Pb 10 INVENTORS MICHAEL C. CAROSELLA JAMES H. JACOBS THOMAS R. McNElLL ATTORN V United States Patent LEAD REMOVAL IN THE ELECTROWINNING or CHROMIUM Michael C. Carosella, Niagara Falls, and James H. Jacobs,

Lewiston, N. Y., and Thomas R. McNeil], Marietta, Ohio, assignors to Union Carbide Corporation, a corporation of New York Application January 27, 1958, Serial No. 111,400

2 Claims. or. 204-405 plication Serial Number 518,464, filed June 28, 1955,

now abandoned.

Lead-alloy anodes and other lead-containingmaterials, such as agitator equipment, coil heaters, lead fittings, and

the like, are widel em 10 ed in a aratus f h y p y pp on e lectm sheet, and the resulting solution 18 filtered to remove the winning of chromium. Such materials serve to contaminate the electrolyte and result in lead contamination of the deposited chromium metal. When electrolytically deposited chromium contaminated with lead is employed in high-chromium alloys, cracking invariably occurs during 3 hot rolling of the alloy. In fact, so critical is the presence of lead impurity in electrolytic chromium, that the manufacturers of high-chromium-alloys tolerate only 0.003 percent maximum lead. i

Accordingly, it is the prime object of the present invention to provide a method for the virtual elimination of lead contamination in solutions employed in the electro winning of chromium.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In accordance with the present invention, chromium sulphate containing solutions to be employed as cell feed in the clectrowinning of chromium are treated by the addition of a barium compound in quantities sufficient to coprecipitate the contained lead impurity.

In the drawing:

Fig. l is a flow sheet of a cyclic process for the electrowinning of chromium as was described in U. S. Patent 2,650,192 entitled Electrowinning of Chromium and issued to M. C. C'arosella et al.;

Fig. 2 is a flow sheet of a lead-removal method embodying the invention for treating solutions in the dissolver step of Fig. 1;

Fig. 3 is a flow sheet of a lead-removal method embodying the invention for treating solutions in the first conditioner step of Fig. 1; and

Fig. 4 is a flow sheet of a lead-removal method embodying the invention for treating solutions from the postcrystallizer filtering step of Fig. 1.

The cyclic process for the electrowinning of chromium described. in U. S. Patent 2,650,192, and shown in Fig. l of the drawing, employs successive leaching, filtering, conditioning, crystallizing, second filtering, ageing, third filtering, and dissolving steps prior to the introduction of the resulting purified solution into the electrolytic cell as cell feed. These steps are indicated as boxes 1, 2, 3, 4, 5, 10, 11, and 12, respectively, of the flow sheet. In addition other purifying steps are accomplished through recycling procedures.

It has been found that the lead removal method of the ICC present invention may be performed in a variety of locations in the purification of the solution; three alternative positions being indicated in Figs. 2, 3, and 4, respectively, of thedrawing.

Referring specifically to Fig. 2 of the drawing, chrome alum crystals are dissolved and a barium compound is added to the resultant solution in sufficient quantity to co-precipitate the contained lead. This step is indicated as box 12 of the flow sheet. taining residue is filtered from the solution in a filter press. This is indicated as box 12a of the flow sheet. The leadfree solution is then fed into the catholyte compartment of the electrolytic cell, which step is indicated as box 13 of the flow sheet.

Referring specifically to Fig. 3 of the drawing, the barium compound is added to the solution resulting from cated as box 3 of the flow sheet, and after passing through the crystallizer, indicated as box 4 of the flow sheet, and filter, the lead and barium precipitate is separated from the solution with the crude iron-ammonium sulphate crystals. The filtering step is indicated as box 5 of the flow sheet.

Referring specifically to Fig. 4 of the drawing, the barium compound is added to the solution reuslting from the first crystallization, indicated as box 5a of the flow barium' and lead containing residue. This filtering step is indicated as box 512 of the fiow sheet. The filtrate so obtained is introduced into the chrome alum ager, indicated as box 10 of the flow sheet.

It has been found that barium compounds may be employed as the precipitant in the purification method of the invention, as for example, barium hydroxide, barium nitrate, and barium chloride. However, the hydroxide is preferably employed due to the fact that it least tends to encourage anode corrosion. The barium compound may be added to the solution in any conventional manner, but its addition as a solution or slurry has been found preferable. In addition, while the purification step may be performed at any of the three positions in the cycle set forth hereinabove, it has been found preferable to perform the purification step just prior to entry of the solution into the cell circuit, since, by doing so, lead pick-up during the process is co-precipitated at this point and the cell feed is purified to the utmost immediately prior to introduction into the cell.

It has been determined that the barium compound reacts with the leach liquor to form barium sulphate which co-precipitates with the lead impurity also in the form of a precipitate. The exact nature of the reaction is not known; however, it is believed that the barium sulphate acts as-a gathering agent for the lead sulphate also present in the solution.

It should be further noted that, while lead sulphate is only slightly soluble in either water or dilute sulphuric acid solutions, even this slight degree of solubility causes a lead contamination of the cell feed that must be substantially reduced for satisfactory electrolytic chromium production. The effectiveness of the process may be clearly seen from the examples.

In an example of the lead-removal method of the invention, 2.0 grams of hydrated barium hydroxide were added as a slurry to a 10 liter sample of cell feed comprising dissolved chrome alum of the composition (NH4)2SO 'CI2(SO4)3'24H2O. Thfi resulting slurry was agitated for one-half hour at C. and subsequently filtered. It was found that the solution, containing 0.043 gram per liter of lead impurity before treatment, was reduced in lead impurity concentration after treatment to 0.0078 gram per liter.

A similar addition of hydrated barium hydroxide in The barium and lead-conslurry form was also made to two one-liter samples of first iron filtrate solution, such as indicated in .Fig. 4 of the drawing. The samples were agitated for one-half hour at 30 C. and 80 C., respectively, and subsequently filtered. In both cases the solution before treatment had a lead concentration of 0.016 gram perliter and after treatment at 30 C. lead impurity concentration was 0.0015 gram per liter, while after treatmentat 80 C. the lead impurity concentration was foundto be 0.0012 gram per liter.

In a further example of the lead removal method of the invention, 200 pounds of hydrated barium hydroxide, as a slurry, were added to a 10,000-gallon batch of. dissolved chrome alum cell feedin. the manner. described in Fig. 2 of the drawing. The treated solutionwas-agitated for four hours and filtered through a plate and frame filter press. A sample of the resultant solution contained only 0.0006 gram per liter of lead and subsequent use of the solution as catholyte in the electro- Winning of chromium resulted in deposited chromium containing an amount of lead never in excess of 0.002 percent by weight.

Prior to the barium hydroxide treatment the chromium metal electrolytically produced had a lead impurity content ranging from 0.010% to 0.016%. By the method of the invention all the solution in the circulating catholyte circuit was treated with two grams per liter barium hydrate and filtered to remove the lead. The catholyte in every cell was filtered through a rubber-coated filter press for two hours to clarify the solution of precipitated lead which had accumulated in the bottom of the cells. All cell feed was also treated with barium hydrate and filtered before advancing to the cell circuit. This method of treatment of all cell solutions except the anolyte and continued treatment of all feed to the cells resulted in continuous production of electrolytic chromiummetal containing less than 0.003% lead, as shown in thefollow ing table:

Table I ssatst fi atl tai.

While the lead-removal method of the present invention has been described hereinabove in connection with the cyclic process for the electrowinning of chromium described in U. S. Patent 2,650,192, it is to be under stood that this method can be used with equal effectiveness in any cyclic process for the electrowinning of chromium employing a chromium-containing sulphate solutionas cell feed.

What is claimed is:

1. In the electrowinning of chromium from chromium sulphate containing solution, the improvement which comprises removing minor amounts of lead impurities from said solution prior to the electrolysis of said solutions by treating said solution with at least one barium compound in suflicient quantity to co-precipitate from said solution said contained lead as lead sulphate.

2..The method in accordance with claim 1, wherein saidbarium compound is barium hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,109,755 Sessions Mar. 1, 1938 FOREIGN PATENTS 2,459 Germany June. 6, 1890 

1. IN THE ELECTROWINNING OF CHROMIUM FROM CHROMIUM SULPHATE CONTAINING SOLUTION, THE IMPROVEMENT WHICH COMPRISES REMOVING MINOR AMOUNTS OF LEAD IMPURITIES FROM SAID SOLUTION PRIOR TO THE ELECTROLYSIS OF SAID SOLUTIONS BY TREATING SAID SOLUTION WITH AT LEAST ONE BARIUM COMPOUND IN SUFFICIENT QUANTITY TO CO-PRECIPITATE FROM SAID SOLUTION SAID CONTAINED LEAD AS LEAD SULPHATE. 